Toolbar: Un-deprecate the `WP_Admin_Bar::add_menu()` method.

This is only a wrapper for the `add_node()` method, but it's in widespread use both in core until [46642] and in thousands of plugins and themes. Deprecating it would have made sense when #19647 was originally opened but that's no longer the case.

Props whyisjake

Fixes #19647 

Built from https://develop.svn.wordpress.org/trunk@46678


git-svn-id: https://core.svn.wordpress.org/trunk@46478 1a063a9b-81f0-0310-95a4-ce76da25c4cd